Fuel dispensers that are used at gas stations for filling fuel tanks of motor vehicles commonly comprise an underground fuel tank, a pump unit, a flow meter, a hose, and an outlet nozzle. The pump unit pumps fuel from the underground tank creating a flow of fuel through the flow meter, the hose and the outlet nozzle. The user of the fuel dispenser utilizes the outlet nozzle to fill the vehicle's fuel tank by pressing in a handle in the nozzle grip.
A fluid meter type commonly used for fuel dispensers is shown by Ainsworth, U.S. Pat. No. 2,756,726 and Spalding, U.S. Pat. No. 5,686,663 and WO 98/49530. In these disclosures a meter having a multiple piston hydraulic motor is used. Fluid is allowed to enter cylinders and cause reciprocation of the pistons. The pistons are connected to a shaft that will rotate as an effect of the reciprocation. A rotary valve, coupled to the shaft, admits liquid to the cylinders or permits flow to the outlet connections, in proper timed relation. The fluid meter utilizes what may be termed “hypothetical” cylinders, mechanically and hydraulically cooperating with the cylinders and pistons which are structurally existent. This is accomplished by arranging the ports and the rotary valve so as to sequentially admit fluid to both the crankcase and the ends of the cylinders at the same time as fluid is withdrawn from the cylinders. The fluid volume admitted to, or withdrawn from, the crankcase is the algebraic sum of the volume withdrawn from, or admitted to, the cylinders. Two pistons, actuated through the valve mechanism, advantageously 120 degrees out of phase, thus perform the work equivalent of three pistons. This reduces the actual number of cylinders required for a given capacity, reduces internal friction and pulsation, and achieve smoother operation. The two pistons are attached via connecting rods to a crankshaft with a radially offset crank pin. The crank pin engages a yoke slot in each connecting rod so that the reciprocating movement of the two pistons is transformed into a rotary motion of the crankcase in accordance with the Scotch Yoke type principle.
When using a fuel dispensing unit and releasing the nozzle handle, for example because the user has decided to stop the dispensing operation, the fuel flow will suddenly be stopped. Modern fuel dispensing units also commonly have a nozzle with a handle that can be locked in dispensing mode so that a user will not have to press down the handle during the entire dispensing operation. These fuel nozzles are normally also equipped with an automatic stop function that will stop dispensing fuel when the nozzle sense that the tank is full by simply releasing the locked handle, to abruptly stop the fuel flow.
A fuel dispensing system, as presented above, is usually a pressurized system with a pressure above atmospheric pressure of about 2-4 bar. The operating pressure will supply a reasonably high flow when the system nozzle valve is open for fuel dispensing. When the nozzle valve is abruptly closed, however, a pressure peak will propagate through the system. This pressure peak can be as high as 40 bar and will increase the wear of the components. This wear of the components due to the pressure peak is a usual cause of broken components in a fuel dispenser leading to a leaking or malfunctioning dispensing unit. A component that can be damaged by a high pressure peak is, for example, the flow meter. A leaking or broken flow meter can cause the dispensing unit to deliver the wrong readings of dispensed fuel. If the pressure wave propagates further than to the fuel pump, other fuel dispensing units in a possible fuel dispensing unit cluster can also be affected by the pressure peak.
A broken fluid meter will not only lead to malfunctioning dispensing units, but will cause additional costs in maintenance when the broken component is replaced. A leaking dispensing unit is also a fire safety issue and a health issue for users of the dispensing unit. Leaks should thus preferably be avoided all together.
One way to handle the problem of high pressure peaks in fuel dispensing units is to make the fuel hose so flexible that the pressure transient is absorbed by deformation in the hose. This, however, leads to a soft hose that is less rugged and will have a reduced lifetime. The hose could also more easily be flattened if squeezed in the dispenser or by a foot. It is rare to have a hose that is flexible enough to absorb the pressure transient sufficiently so that the remaining pressure variation is not harmful for other components in the fuel dispensing unit.
It is also known in general pressurized fluid systems to have expansion volumes/balance volumes to absorb pressure variations.